tests: Fix callback arguments in fake-document-portal
tests: Fix callback arguments in fake-document-portal

They didn’t match the prototype generated by `gdbus-codegen`, which
meant that the FD list was being iterated incorrectly. Secondly, the
document ID list returned by the method was not NULL terminated, which
could lead to reading off the end of the list.

Somehow, neither of these bugs caused problems on Linux, but they did
cause problems on FreeBSD.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1983

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib